Wobble-type centrifuges normally have a foraminous drum that is mounted on an at least partially flexible core shaft having a lower part lying on a first axis and an upper part lying on a second axis intersecting the first axis at an acute angle. A tube shaft surrounds and journals this core shaft and has lower and upper parts, respectively, centered on the first and second axes. The tube shaft is rotated about the first or lower axis and the core shaft is rotated at a different anuglar speed so that the foraminous drum wobbles or nutates slightly as it rotates. The effect of this operation, as described in German patent publications No. 1,072,199, 1,198,295, 1,288,990, as well as Swiss patent 566,816 and commonly assigned U.S. patent application Ser. No. 884,162 filed Mar. 7, 1978, is that the solid phase of a suspension being centrifuged is caused to move axially along the drum which to this end is given an upward or downward taper. Hereinafter the terms "upper" and "lower" and associated terminology are used purely for convenience's sake, there being no intention to limit the invention to a particular orientation of the parts relative to the vertical.
Three factors normally control the displacement of the solid phase of the suspension being centrifuged:
1. The varying angle of the centrifugal force with respect to the surface of the drum due to the wobbling of the generator.
2. The oscillatory displacement parallel to the rotation axes of the surface of the drum.
3. The Coriolis acceleration perpendicular to the machine axis whose level is proportional to the distance from the intersection of the two rotation axes.
No matter what orientation the machine has relative to the vertical the considerable centrifugal forces are therefore effective to displace the solid phase of the solution from the small end toward the wide end of the drum in the form of a continuously moving filter cake whose depth decreases in the direction of movement. As the thickness of the filter cake decreases, the filtering effect is lessened and the residence time of the solid phase inside the centrifuge decreases, so that the operation is correspondingly less effective at extracting the liquid phase from the suspension. This inefficiency is even greater in the known wobble centrifuges since the distance between the location at which the suspension is fed in and the intersection of the two axes is smaller than the distance between this intersection and the edge of the drum over which the solid phase falls when finally leaving the drum.